Hydrogen storage alloy composition and electrode using said alloy composition

ABSTRACT

A hydrogen absorbing alloy composition for a nickel-hydrogen secondary battery which includes LnNi 5  hydrogen absorbing alloy, where Ln represents at least one rare-earth element. The hydrogen absorbing alloy composition also includes at least one compound selected from the group consisting of heavy rare-earth oxides, heavy rare-earth hydroxides, compound oxides including at least one rare-earth element and compound hydroxides including at least one rare-earth element. Rare-earth elements can be selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Exemplary hydrogen absorbing alloy compositions include La alone or in combination with one other rare-earth element, such as Ce, Pr, Nd, or Sm. Exemplary rare-earth oxides include Yb 2  O 3 , Er 2  O 3  and GdO 3  and exemplary rare-earth hydroxides include Yb(OH) 3  and Er(OH) 3 .

FIELD OF THE INVENTION

The present invention relates to a hydrogen storage alloy compositionand, more particularly, to a hydrogen storage alloy composition whichhas a high capacity and a long life upon repetition of alternateabsorption and desorption of hydrogen and is therefore used suitably inthe hydrogen absorbing alloy-utilized field, e.g., for a hydrogenstorage tank and a negative electrode of a nickel-hydrogen battery, andfurther to a hydrogen absorbing alloy electrode for a nickel-hydrogenstorage battery.

BACKGROUND OF THE INVENTION

Since alloys capable of absorbing and desorbing hydrogen, or hydrogenstorage alloys, were discovered, they have been applied to not only ahydrogen storage means but also a battery and the like. In particular,the hydrogen absorbing alloys developed with the intention of using themfor a hydrogen car, an air conditioner and so on have undergone variousimprovements in recent years.

More specifically, a LaNi₅ alloy which is first examined as a hydrogenabsorbing alloy (See Japanese Tokkai Sho 51-13934, wherein the term"Tokkai" as used herein means an "unexamined published patentapplication") has the advantage of a high hydrogen storage capacity, butit suffers from disadvantages that not only La metal is expensive butalso it is pulverized easily by repeating the absorption and desorptionof hydrogen alternately and apt to be corroded by an alkaline or acidicsolution.

Accordingly, when it is used as the electrode of an alkaline secondarybattery, such an alloy can ensure a high initial electric capacity inthe secondary battery. However, the electric capacity of the secondarybattery is reduced to one-half or below by the charge-and-dischargecycles repeated about 50 times. Thus, such a secondary battery cannotstand a long use.

The aforementioned drawbacks of the LaNi₅ alloy are mitigated byreplacing a part of the La element by another rare earth element, suchas Ce, Pr or Nd, and/or a part of the Ni element by another metalelement such as Co, Al or Mn (See, e.g., Japanese Tokkai Sho 53-4918,Sho 54-64014, Sho 60-250558, Sho 61-91862 and Sho 61-233969).

The LaNi₅ alloys modified as described above (hereinafter referred to as"LaNi₅ type hydrogen absorbing alloys"), though they are somewhatinferior to the LaNi₅ alloy in hydrogen storage capacity, have anadvantage over the LaNi₅ alloy in being improved in corrosion resistanceto an alkaline or acidic solution. When they are each used for thenegative electrode of an alkaline secondary battery, they can thereforelengthen the charge-and-discharge cycle life of the alkaline secondarybattery. However, such a prolonged cycle life of the alkaline secondarybattery, which is obtained by the use of the foregoing LaNi₅ typehydrogen absorbing alloys, is still insufficient, and the electriccapacity per unit weight is also unsatisfactory.

High capacity (or high electric capacity per unit weight) and longlifetime are the characteristics generally required for a battery. Thus,reduction in the reserve quantity of an electrode becomes necessary forthe production of a secondary battery having a high capacity by the useof a LaNi₅ type hydrogen absorbing alloy. Although the capacity of abattery can be elevated by the reduction of the reserve quantity, it isattended by a problem of shortening the cycle life. This problem is notyet solved.

When the secondary battery is overcharged, the oxygen gas generated fromthe positive electrode promotes the oxidation of a hydrogen absorbingalloy, and thereby the charge acceptance of the hydrogen absorbing alloyis lowered. As a consequence of the foregoing, hydrogen gas comes to beproduced upon charging, and the hydrogen gas produced raises theinternal pressure of the closed secondary battery to actuate a pressurevalve, thereby causing a loss of the electrolytic solution. Thus, theinternal resistance of the battery is increased; as a result, thedischarge capacity is lowered as the charge-and-discharge cycle isrepeated.

For the purpose of removing the foregoing drawback, the method ofetching the hydrogen absorbing alloy with an acidic or alkaline solutionand the method of plating the hydrogen absorbing alloy with copper ornickel have been proposed.

However, those methods are unsuccessful in the prevention of corrosionat the active surfaces attributable to cracks the hydrogen absorbingalloy newly has in it upon repetition of alternate charge and discharge,so that they cannot inhibit the alloy to lower its hydrogen storagecapacity. Thus, it is difficult for those methods also to ensure asufficiently long charge-and-discharge cycle life in the electrode.

With the intention of obviating the foregoing defect, the method oflengthening the charge-and-discharge cycle life of a battery has beenproposed (Japanese Tokkai Hei 6-215765), wherein yttrium and/or ayttrium compound (inclusively referred to as "yttrium" hereinafter) isincorporated in a hydrogen absorbing alloy electrode; as a result, theyttrium is dissolved in an alkali electrolyte and deposited on activesurfaces which are newly formed in the hydrogen absorbing alloy due tothe generation of cracks, and the yttrium cover thus formed on theactive surface inhibits the hydrogen absorbing alloy from undergoingoxidation to prevent the lowering of the hydrogen storage capacity. Inaddition, the method of incorporating a light rare-earth oxide in ahydrogen absorbing alloy in place of the foregoing yttrium compound hasbeen proposed (Japanese Tokkai Hei 8-222210). However, the former method(Japanese Tokkai Hei 6-215765) is attended by deterioration in theinitial activity, while the latter method (Japanese Tokkai Hei 8-222210)has a defect of being ineffective for hydrogen absorbing alloys otherthan those having laves phases with respect to the charge-and-dischargecycle life and the high temperature storage characteristics.

SUMMARY OF THE INVENTION

As a result of our intensive studies of the aforementioned problems, ithas been found that, when at least either a heavy rare-earth oxide or aheavy rare-earth hydroxide, or at least either a compound oxidecomprising at least one rare earth element or a compound hydroxidecomprising at least one rare earth element is incorporated in a hydrogenabsorbing alloy electrode, not only the foregoing defect can be removedbut also the charge-and-discharge cycle life can be prolonged to asatisfactory extent, thereby achieving the present invention.

Therefore, a first object of the present invention is to provide ahydrogen absorbing alloy composition suitable for a negative electrodeof a high-performance nickel-hydrogen secondary battery.

A second object of the present invention is to provide a hydrogenabsorbing alloy electrode for a nickel-hydrogen secondary battery whichhas a high capacity and a long charge-and-discharge cycle life but alsoimproved initial activity and corrosion resistance.

The above-described objects of the present invention are attained with ahydrogen absorbing alloy composition which comprises;

(1) 100 parts by weight of a LnNi₅ hydrogen absorbing alloy, wherein Lnrepresents at least one rare earth element, and

(2) 0.2 to 20 parts by weight of at least one compound selected from thegroup consisting of heavy rare-earth oxides, heavy rare-earth hydroxides(the term "heavy rare-earth" is defined hereinafter), compound oxidescomprising at least one rare earth element and compound hydroxidescomprising at least one rare earth element:

and a hydrogen absorbing alloy electrode for a nickel-hydrogen secondarybattery, wherein the aforesaid composition and a conductive support areused.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the x-ray diffraction spectrum of a mixture of Yb₂ O₃, Y₂O₃ and Er₂ O₃ powders, which is one example of rare-earth oxidemixtures, and the peak assignment thereof.

FIG. 2 shows the X-ray diffraction spectrum of a compound oxide powderproduced from Yb₂ O₃, Y₂ O₃ and Er₂ O₃, which is one example of compoundoxides according to the present invention, and the peak assignmentthereof.

DETAILED DESCRIPTION OF THE INVENTION

The Ln in the rare-earth alloy represented by formula LnNi₅, which isused as the component (1) in the present invention, includes at leastone rare earth element, and has no particular restriction as toconstituent elements thereof. More specifically, any of rare earthelements known to be usable for hydrogen absorbing alloys can beemployed as Ln.

Examples of a rare earth element which can be suitably used in thepresent invention include La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb and Lu.

In particular, it is advantageous to mainly use La as the Ln and furtheruse Ce, Pr, Nd or/and Sm in combination with La.

The LaNi₅ type hydrogen absorbing alloys which can used as the component(1) in the present invention, are represented by LaNi₅ in terms ofstoichiometric ratio. Preferably, such a LaNi₅ type hydrogen absorbingalloy is an intermetallic compound prepared by replacing a part of La byanother rare earth element, such as Ce, Pr or Nd and further replacing apart of Ni by other metals, such as Co, Mn, Al, Fe and/or Cu. From theviewpoint of achieving a satisfactory cycle life, it is desirable toreplace a part of Ni by at least Mn, preferably Mn and Al, morepreferably Mn, Al and Co.

More specifically, such LaNi₅ type hydrogen absorbing alloys, which arefavorably used in the present invention, are represented byLn(Ni(w-x-y-z)Mn_(x) Al_(y) Co_(z)), wherein Ln is La alone or a mixtureof La and another rare earth element, preferably Ce, Pr, Nd, Sm or amixture thereof, and w, x, y and z are in the following rangesrespectively; 4.8≦w≦5.3, 0<x≦0.6, 0<y≦0.5, and 0<z≦1.0.

As an example of such intermetallic compounds, mention may be made ofthe LaNi₅ type alloy in which 20 weight % of La is replaced by Ce, 14atomic % of Ni is replaced by Co, 4 atomic % of Ni is replaced by Mn and6 atomic % of Ni is replaced by Al. When the values of w, x, y and z areout of the foregoing ranges, the hydrogen absorbing-and-desorbing cyclelife is shortened and the hydrogen storage capacity is decreased in somecases.

Further, it is desirable for the hydrogen absorbing alloy used in thepresent invention to have an average grain diameter (D) of from 1 to300μ.

The heavy rare-earth oxide and the heavy rare-earth hydroxide, whicheach or both can be used as the component (2), are represented by R¹ ₂O₃ and R¹ (OH)₃ respectively, wherein R¹ is a heavy rare-earth element.The term "heavy rare-earth element" used herein is intended to includeEu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

With respect to such oxides and hydroxides, it is desirable that R¹ beYb, Er, Dy, Gd or Ho. In particular, ytterbium oxide (Yb₂ O₃), ytterbiumhydroxide (Yb(OH)₃), erbium oxide (Er₂ O₃) and erbium hydroxide(Er(OH)₃) are preferred over the others.

When the heavy rare-earth oxide as described above is added to ahydrogen absorbing alloy, it is desirable that the specific surface areathereof be from 0.1 to 10 m² /g, preferably from 0.2 to 5.0 m² /g,determined by BET method. When the specific surface area of the oxideused is in the aforesaid range, the resultant alloy can have an improvedalkali resistance.

When the heavy rare-earth hydroxide as described above is added to ahydrogen absorbing alloy, it is desirable for the rare-earth metalpresent therein to have a purity of at least 30 wt %, preferably from 30to 80 wt %, from the viewpoint of the reactivity with the hydrogenabsorbing alloy.

Besides the foregoing heavy rare-earth oxides and hydroxides, thecompound oxides and the compound hydroxides which each comprise thecombination of at least two different rare earth elements can be used asthe component (2) of the present composition.

Such a compound oxide (compound hydroxide) is not a simple mixture ofoxides (hydroxides) but a solid solution of oxides (hydroxides). And itcan be produced, e.g, in the following manner: Starting materials (whichincludes at least one rare-earth compound) for the desired compoundoxide or compound hydroxide are mixed in respectively intended amounts,and dissolved in a water solution of nitric acid (or hydrochloric acid,sulfuric acid, hydrofluoric acid or the like). The resultant solution isadmixed with oxalic acid (or an oxalate, such as potassium oxalate,sodium oxalate or ammonium oxalate, a carbonate, such as K₂ CO₃, (NH₄)₂CO₃ or Na₂ CO₃, or an alkali, such as KOH, NH₄ OH or NaOH) withstirring, thereby causing coprecipitation. The coprecipitated productwas filtered off, rinsed with water, and then sintered at about800°-1,100° C. (in the atmosphere).

The production of a compound oxide (compoound hydroxide), or a solidsolution, can be confirmed by X-ray powder method. More specifically,each of the compound oxides (compound hydroxides) used in the presentinvention, which comprises at least two different metals including atleast one rare earth metal, shows a diffraction spectral pattern havingno split in peaks (as shown in FIG. 2), while the diffraction spectralpattern of the mixture comparable thereto has split in each peak (asshown in FIG. 1); thereby confirming that each compound oxide (compoundhydroxide) has its own crystal structure.

The rare earth elements comprised in each of the compound oxides andeach of the compound hydroxides, which can be used as the component (2)of the present composition, can be any of the combination of two or morerare earth elements selected from the group consisting of La, Ce, Pr,Nd, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

Examples of a compound oxide which can be used include the combinationsof at least two different oxides selected from the group consisting ofY₂ O₃, Yb₂ O₃, La₂ O₃, Ce₂ O₃, Nd₂ O₃, Pr₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃,Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃ and Lu₂ O₃. Such a compound oxideis represented by (R² ₂ O₃)_(a).(R³.sbsp.2 O₃)_(b), (R² ₂O₃)_(c).(R³.sbsp.2 O₃)_(d).(R⁴.sbsp.2 O₃)_(e) or the like, wherein R²,R³ and R⁴ are different from one another and each represents a rareearth element selected from the above-recited ones, a and b are eachfrom 0.1 to 0.9 by mole, preferably from 0.2 to 0.8 by mole, providedthat a+b is 1 by mole, and c, d and e are each from 0.1 to 0.8 by mole,provided that c+d+e is 1 by mole.

Examples of a compound hydroxide which can be used include thecombinations of at least two different hydroxides selected from thegroup consisting of Y(OH)₃, Yb(OH)₃, La(OH)₃, Ce(OH)₃, Nd(OH)₃, Pr(OH)₃,Sm(OH)₃, EU(OH)₃, Gd(OH)₃, Tb(OH)₃, Dy(OH)₃, Ho(OH)₃, Er(OH)₃, Tm(OH)₃and Lu(OH)₃. Such a compound hydroxide is represented by (R²(OH)₃)_(a).(R³ (OH)₃)_(b), (R² (OH)₃)_(c).(R³ (OH)₃)_(d).(R⁴ (OH)₃)_(e)or the like, wherein R², R³ and R⁴ are different from one another andeach represents a rare earth element selected from the above-recitedones, a and b are each from 0.1 to 0.9 by mole, provided that a+b is 1by mole, and c, d and e are each from 0.1 to 0.8 by mole, provided thatc+d+e is 1 by mole.

When a compound oxide or a compound hydroxide is produced using thecombination of at least 4 different oxides or hydroxides, it is requiredthat the amount of each oxide or hydroxide used be at least 10 mole %and the total amount of oxides or hydroxides used be 100 mole %.

Examples of a compound oxide and a compound hydroxide which can beadvantageously used in the present invention include the compound oxidesproduced using Yb₂ O₃ and Lu₂ O₃ in combination, those produced uing Yb₂O₃ and Er₂ O₃ in combination, those produced using Er₂ O₃ and DY₂ O₃ incombination, those produced using Yb₂ O₃, SM₂ O₃ and Gd₂ O₃ incombination, those produced using Y₂ O₃, Er₂ O₃ and Yb₂ O₃ incombination, the compound hydroxides produced using Yb(OH)₃ and Er(OH)₃in combination, and those produced using Er(OH)₃ and Dy(OH)₃ incombination.

Besides the combination of two or more rare earth elements, thecombination of at least one rare earth element and at least one metalelement except rare earth elements, such as Co, Ni, Zr, Hf, Al, V andNb, can also be comprised in the compound oxides or the compoundhydroxides used as the component (2) in the present invention.

Such compound oxides which can be used are represented by formula R⁵MO_(m), wherein R⁵ is a rare earth element such as Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu or Y, M is a metal element except rare earth elements andm is a number of 2 to 6, with examples including Dy₂ Hf₂ O₇, Eu₂ Hf₂ O₇,Yb₂ Zr₂ O₇, Er₂ Zr₂ O₇, YAlO₄, YNbO₄, YVO₄ and ErAlO₃.

The hydrogen absorbing alloy composition of the present invention isrequired to comprise 100 parts by weight of the component (1) and 0.2-20parts by weight of the component (2). When the amount of component (2)added is less than 0.2 parts by weight, the hydrogen absorbing alloyobtained cannot have sufficient improvements in corrosion resistance andhydrogen absorbing-and-desorbing cycle life characteristics; while, whenit is increased beyond 20 parts by weight, the hydrogen absorbing alloyobtained suffers deterioration in contact characteristics (includingheat conductivity and electric conductivity) and the production costthereof becomes high. Preferably, the amount of the component (2) addedis from 0.5 to 5 parts by weight per 100 parts by weight of thecomponent (1).

In particular, the compound oxides whose metallic constituents are rareearth elements alone are preferred as the component (2).

The aforementioned components (1) and (2) are each ground to a powderwith a ball mill, a jet mill, a pulverizer or the like, and then mixedtogether with conventional stirring and mixing means to obtain a powderycomposition. Therein, it is desirable that the average grain diameter ofthe component (2) be from 1 to 300 μm, preferably from 10 to 100 μm. Thethus obtained hydrogen absorbing alloy composition has a high hydrogenstorage capacity, a long hydrogen absorbing-and-desorbing cycle life anda high corrosion resistance.

The hydrogen absorbing alloy composition of the present invention can beobtained by adding an organic binder as described below to the foregoingpowdery composition.

The electrode of the present invention can be prepared as follows: Thepowdery composition prepared above is added to an aqueous bindersolution, and kneaded to make a paste. The paste obtained is filled intoa three-dimensional conductive support, such as textile Ni or foamed Ni,or a two-dimensional conductive support such as a punching metal, andthen pressed to form a negative electrode for a nickel-hydrogensecondary battery.

The organic binder used for binding the foregoing hydrogen absorbingalloy composition can be properly selected from the binders used forconventional hydrogen absorbing alloy electrodes.

Examples of such a binder include celluloses such as methyl celluloseand carboxymethyl cellulose, polyvinyl alcohol, polyethylene oxide,polytetrafluoroethylene and high-molecular latexes. These binders can beused alone or as a mixture of two or more thereof.

The amount of the organic binder used is from 0.1 to 20 parts by weight,preferably from 0.1 to 6 parts by weight, per 100 parts by weight of ahydrogen absorbing alloy. When the amount of a binder used is increasedbeyond 20 parts by weight, the activity becomes poor and the capacity(maximum capacity) is lowered; while, when it is less than 0.1 parts byweight, the cycle life is shortened and the resultant alloy comes offthe support.

In the hydrogen absorbing alloy electrode according to the presentinvention, the rare-earth metal(s) or compound(s) contained in a LaNi₅type hydrogen absorbing alloy layer is(are) dissolved in an alkalielectrolyte, and deposited on active surfaces which are newly formed inthe hydrogen absorbing alloy layer due to the cracks generated duringcharge-and discharge cycles. The cover of rare-earth element(s) thusformed on the active surface inhibits the hydrogen absorbing alloysurface from undergoing oxidation to ensure a high capacity, a longcharge-and-discharge cycle life and an improved initial activity in theelectrode. Therefore, the electrode of the present invention is wellsuited for a nickel-hydrogen secondary battery.

The present invention will now be illustrated in greater detail byreference to the following examples. However, the invention should notbe construed as being limited to these examples.

EXAMPLE 1

The La--Ce alloy having the La content of 80 weight % and the Ce contentof 20 weight %, Ni, Co, Mn and Al were weighed out in their respectiveamounts such that the atomic ratio of La--Ce to Ni to Co to Mn to Al was1.00:3.80:0.7 0:0.20:0.30 (B/A=5.0), and molten with a high-frequencyfurnace, followed by cooling. Thus, a LaNi₅ type alloy was prepared.This alloy was subjected to a heat treatment at 1000-1100° C. for 5hours in the argon atmosphere, and ground mechanically so that thepowder obtained had an average grain diameter of 40-50 μm or below.

To 16 g of the thus obtained alloy powder, ytterbium oxide having aspecific surface area of 2.2 m² was added in an amount of 2 weight %,and further 4 g of a 3 % solution of polyvinyl alcohol (average degreeof polymerization: 2,000; saponification degree: 98 mole %) was added toprepare a paste.

The paste prepared above was filled homogeneously into a foam nickelhaving a porosity of 95 %, and pressed into a sheet-form hydrogenabsorbing alloy having a thickness of 0.5-1.0 mm. To the surface of thesheet-form alloy obtained, a lead line was attached to make a negativeelectrode.

Additionally, a known foam metal nickel having the capacity of 2,400milliampere-hour (hereinafter referred to as "mAh") was used as positiveelectrode.

Then, a separator made of a nonwoven fabric of polypropylene, which wasrendered hydrophilic by conventional treatments, was put between thesheet-form negative and positive electrodes prepared above were woundinto a roll, and packed into a cylindrical container. Further, anaqueous KOH solution having a concentration of 6 moles per liter waspoured as an electrolytic solution into the container. Then, thecontainer was sealed, thereby making a SC-size closed-typenickel-hydrogen (alkaline) secondary battery having a nominal capacityof 2,400 mAh.

Under the temperature regulated at 20° C., the thus made battery wascharged for 4 hours by sending thereto an electric current of 720milliampere (mA) and then made to discharge an electric current of 480mA until the battery voltage was decreased to 1.0 V. Thischarge-and-discharge cycle operation was repeated at 20° C. During therepeated operations, the number of cycles repeated till the capacity wasdropped to 60 % of the initial capacity was determined, and thereby thecharge-and-discharge cycle life at 20° C. was evaluated. In addition,the internal pressure in the battery was measured after thecharge-and-discharge cycle operation was repeated 10 times.

Further, a negative electrode-regulated open-type nickel-hydrogensecondary battery was made using the negative electrode (3×4 cm²)prepared above, a conventional positive electrode made of sintered Niand the same separator and electrolytic solution as used for making theforegoing closed-type battery, and the initial capacity (the capacity atthe tenth cycle) thereof was measured as follows:

The battery was charged for 5 hours at a charging rate of 0.3 Coulomb(C) under a temperature regulated at 20° C., and then discharged at adischarging rate of 0.2 C under a temperature regulated at 20° C. tillthe battery voltage was dropped to 0.8 V. This charge-and-dischargecycle was repeated 10 times. The initial capacity was evaluated by thecapacity measured at the tenth cycle.

The results obtained are shown in Table 1.

EXAMPLES 2 AND 3

Alloy compositions were prepared in the same manner as in Example 1,except that Er₂ O₃ and Gd₂ O₃ were each used in place of Yb₂ O₃, andevaluated by the same method as adopted in Example 1. The resultsobtained are also shown in Table 1.

EXAMPLES 4 AND 5

Alloy compositions were prepared in the same manner as in Example 1,except that the hydroxides, Yb(OH)₃ and Er(OH)₃, were each used in placeof the oxide Yb₂ O₃, and evaluated by the same method as adopted inExample 1. Additionally, the ytterbium hydroxide used had a ytterbiumpurity of 35.3 %. The results obtained are also shown in Table 1.

EXAMPLE 6

The La--Ce alloy having the La content of 80 weight % and the Ce contentof 20 weight %, Ni, Co, Mn and Al were weighed out in their respectiveamounts such that the atomic ratio of La--Ce to Ni to Co to Mn to Al was1.00:3.90:0.7 0:0.20:0.30 (B/A=5.1), and molten with a high-frequencyfurnace, followed by cooling. Thus, a LaNi₅ type alloy was prepared.This alloy was subjected to a heat treatment at 1,000-1,100° C. for 5hours in the argon atmosphere, and ground mechanically so that thepowder obtained had an average grain diameter of 40 μm or below.

To 16 g of the thus obtained alloy powder, a compound oxide produced inthe manner described below was added in an amount of 3 weight %, andfurther 4 g of a 3 % polyvinyl alcohol solution was added to prepare apaste. The compound oxide used was produced as follows: Ytterbium oxideand lutetium oxide are mixed in a ratio of 1:1 by weight, dissolved in anitric acid solution, admixed with oxalic acid with stirring to causecoprecipitation, filtered off, rinsed with water, and then sintered atabout 900° C. in the atmosphere.

The thus prepared alloy composition was evaluated by the same method asadopted in Example 1. The results obtained are also shown in Table 1.

EXAMPLES 7 AND 15

Other five compound oxides were produced using two or three differentrare-earth oxides in the ratios shown in Table 1 respectively.

Alloy compositions were prepared in the same manner as in Example 6,except that those compound oxides were each used in place of thecompound oxide used in Example 6, and evaluated by the same method asadopted in Example 1. The results obtained are also shown in Table 1.

EXAMPLES 16 AND 18

Compound hydroxides were produced using two different rare-earthhydroxides in the ratios (by weight) shown in Table 1 respectively.

Alloy compositions were prepared in the same manner as in Example 6,except that those compound hydroxides were each used in place of thecompound oxide used in Example 6, and evaluated by the same method asadopted in Example 1. The results obtained are also shown in Table 1.

COMPARATIVE EXAMPLES 1 TO 4

Alloy compositions were prepared in the same manner as in Example 1,except that La₂ O₃ was added in place of Yb₂ O₃ (Comparative Example 1)and the amounts of Yb₂ O₃ added were 0, 0.05 weight % and 30.0 weight %(Comparative Examples 2, 3 and 4) respectively. The evaluation of thesecompositions were made using the same method as adopted in Example 1.The results obtained in Comparative Examples 1 and 2 are shown in Table1, and those in Comparative Examples 3 and 4 are shown in Table 3.

EXAMPLES 19 TO 23 AND COMPARATIVE EXAMPLE 5

The La--Ce alloy having the La content of 60 weight % and the Ce contentof 40 weight %, Ni, Co, Mn and Al were weighed out in their respectiveamounts such that the atomic ratio of La--Ce to Ni to Co to Mn to Al was1.00:3.70:0.7 0:0.20:0.30 (B/A=4.9), and molten with a high-frequencyfurnace, followed by cooling. Thus, a LaNi₅ type alloy was prepared.This alloy was subjected to a heat treatment at 1000-1100° C. for 5hours in the argon atmosphere, and ground mechanically so that thepowder obtained had an average grain diameter of 40-50 μm or below.

To 16 g of the thus obtained alloy powder, a ytterbium oxide having aspecific surface area of m² (Example 19), 5.5 m² (Example 21), or 13.5m² (Example 22), or a ytterbium hydroxide wherein the purity ofytterbium contained was 35.3 % (Example 20) or 21.3 % (Example 23) wasadded in an amount of 2 weight %.

The thus prepared hydrogen absorbing alloy compositions and the hydrogenabsorbing alloy composition prepared in the same manner as in Example19, except that neither rare-earth oxide nor rare-earth hydroxide wasadded to the alloy (Comparative Example 5), were each immersed in anaqueous KOH solution having a concentration of 6 mol/l for 72 hours at80° C., and then subjected successively to filtration, rinsing anddrying operations. The quantity of acicular rare-earth hydroxideproduced on the thus treated alloy surface (corrosion quantity) wasdetermined by X-ray powder method, and thereby the corrosion resistanceof each alloy composition was evaluated.

More specifically, a drop of silicone oil was added to a 0.2 g portionof each sample and kneaded thoroughly. It was added little by little tothe filling part of a glass sample plate (depth :0.2 mm), and pressedwith a cover glass for leveling. The thus prepared sample plate wassubjected to X-ray diffraction analysis. Therein, the measurement wasperformed using 50 kV-200 mA X-ray under a condition that X-raydiffraction angle was changed from 37.50 to 41.50 by stepwise scanningwith a step gap of 0.04° and the counting time was 30 seconds. The thusmeasured (201) diffraction peak was submitted to smoothing andbackground-subtracting procedures, and then the integrated area thereof(referred to as "peak intensity hereinafter) was calculated. Theevaluation results are shown in Table 2. The corrosion quantities setforth therein are relative values, with the peak intensity in the caseof Comparative Example 5 being taken as 100.

Further, the hydrogen absorbing alloy compositions prepared aboveunderwent the same life test as in Example 1. The results obtained arealso shown in Table 2.

EXAMPLES 24 TO 27

To the same alloy powder as prepared in Example 1, the same ytterbiumoxide used in Example 1 was added in a different proportion, from 0.005to 30 weight %. The hydrogen absorbing alloy compositions thus obtainedwere tested by the same methods as in Example 1. The results obtainedare shown in Table 3.

As can be seen from Tables shown below, the hydrogen absorbing alloycompositions according to the present invention were markedly reduced inquantity of hydroxides produced by the corrosion of the alloy surface,and successful in achieving considerably prolonged charge-and-dischargecycle life and great reduction in internal pressure of a secondarybattery.

                                      TABLE 1                                     __________________________________________________________________________                                 Charge and discharge                                                                    Internal Pressure                                                                      Initial Capacity              Sample                                                                              Compound added         cycle life                                                                              of battery (kg/cm.sup.2)                                                               (mAh/g)                       __________________________________________________________________________    Example 1                                                                           Yb.sub.2 O.sub.3       400       3.5      230                           Example 2                                                                           Er.sub.2 O.sub.3       450       3.0      220                           Example 3                                                                           Gd.sub.2 O.sub.3       360       4.0      210                           Example 4                                                                           Yb(OH).sub.3           420       3.5      200                           Example 5                                                                           Er(OH).sub.3           430       3.0      210                           Example 6                                                                           Compound Oxide produced using Yb.sub.2 O.sub.3 and Lu.sub.2                                          820ub.3   2.5      240                                 in the ratio of 1:1 by weight                                           Example 7                                                                           Compound Oxide produced using Yb.sub.2 O.sub.3 and Er.sub.2                                          859ub.3   2.5      235                                 in the ratio of 1:2 by weight                                           Example 8                                                                           Compound Oxide produced using Er.sub.2 O.sub.3 and Dy.sub.2                                          850ub.3   3.5      240                                 in the ratio of 1:1 by weight                                           Example 9                                                                           Compound Oxide produced using Er.sub.2 O.sub.3 and                                                   500.sub.2 3.5      200                                 in the ratio of 1:1 by weight                                           Example 10                                                                          Compound Oxide produced using Er.sub.2 O.sub.3 and                                                   820ub.2 O.sub.3                                                                         2.5      240                                 in the ratio of 9:1 by weight                                           Example 11                                                                          Compound Oxide produced using Yb.sub.2 O.sub.3 and Er.sub.2                                          650ub.3   2.5      200                                 in the ratio of 9:1 by weight                                           Example 12                                                                          Compound Oxide produced using Yb.sub.2 O.sub.3, Sm.sub.2 O.sub.3              and                    750       2.5      235                                 Gd.sub.2 O.sub.3 in the ratio of 8:1:1 by weight                        Example 13                                                                          Compound Oxide produced using Yb.sub.2 O.sub.3, Er.sub.2 O.sub.3              and                    800       2.5      235                                 Yb.sub.2 O.sub.3 in the ratio of 7:2:1 by weight                        Example 14                                                                          Compound Oxide produced using Y.sub.2 O.sub.3, Er.sub.2 O.sub.3                                      650       3.0      230                                 Yb.sub.2 O.sub.3 in the ratio of 1:8:1 by weight                        Example 15                                                                          Compound Oxide produced using Dy.sub.2 O.sub.3, Er.sub.2 O.sub.3              and                    650       3.5      240                                 Gd.sub.2 O.sub.3 in the ratio of 8:1:1 by weight                        Example 16                                                                          Compound Oxide produced using Yb(OH).sub.3 and                                                       750       2.5      240                                 Lu(OH).sub.3 in the ratio of 1:1 by weight                              Example 17                                                                          Compound Oxide produced using Yb(OH).sub.3 and                                                       750       2.5      235                                 Er(OH).sub.3 in the ratio of 1:2 by weight                              Example 18                                                                          Compound Oxide produced using Er(OH).sub.3 and                                                       500       3.5      240                                 Dy(OH).sub.3 in the ratio of 2:3 by weight                              Comparative                                                                         La.sub.2 O.sub.3       250       12.5     235                           Example 1                                                                     Comparative                                                                         not added              250       12.5     235                           Example 2                                                                     __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________             Compound                                                                            BET RE purity                                                                          corrosion                                                                          Charge and discharge                             Sample   added (m.sup.2 /g)                                                                      (%)  quantity                                                                           cycle life                                       __________________________________________________________________________    Example 19                                                                             Yb.sub.2 O.sub.3                                                                    2.2 --   61   400                                              Example 20                                                                             Yb(OH).sub.3                                                                        --  35.3 57   420                                              Example 21                                                                             Yb.sub.2 O.sub.3                                                                    5.5 --   85   350                                              Example 22                                                                             Yb.sub.2 O.sub.3                                                                    13.5                                                                              --   72   300                                              Example 23                                                                             Yb(OH).sub.3                                                                        --  21.3 65   280                                              Compar. Example 5                                                                      not added                                                                           --  --   100  250                                              __________________________________________________________________________     RE stands for rare earth element.                                        

                                      TABLE 3                                     __________________________________________________________________________             Compound                                                                            Amount Charge and discharge                                                                    Internal Pressure                                                                      Initial Capacity                     Sample   added added (Wt %)                                                                         cycle life                                                                              of battery (kg/cm.sup.2)                                                               (mAh/g)                              __________________________________________________________________________    Example 24                                                                             Yb.sub.2 O.sub.3                                                                    0.5    400       3.5      230                                  Example 25                                                                             Yb.sub.2 O.sub.3                                                                    1.0    400       3.5      230                                  Example 26                                                                             Yb.sub.2 O.sub.3                                                                    5.0    380       3.5      210                                  Example 27                                                                             Yb.sub.2 O.sub.3                                                                    20.0   350       5.5      180                                  Compar. Example 3                                                                      Yb.sub.2 O.sub.3                                                                    0.05   250       12.5     235                                  Compar. Example 4                                                                      Yb.sub.2 O.sub.3                                                                    30.0   150       14.5     235                                  __________________________________________________________________________

What is claimed is:
 1. A hydrogen absorbing alloy composition whichcomprises;(1) 100 parts by weight of a LnNi₅ type hydrogen absorbingalloy, wherein Ln represents at least one rare earth element, and (2)0.2 to 20 parts by weight of at least one rare-earth containingcomponent selected from the group consisting of:oxides of heavyrare-earth elements Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; hydroxides ofheavy rare-earth elements Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;compound oxides comprising at least two oxides of different metalsincluding at least one rare-earth element; and compound hydroxidescomprising at least two hydroxides of different metals including atleast one rare-earth element.
 2. A hydrogen absorbing alloy compositionaccording to claim 1, wherein the rare-earth compound as the component(2) is at least one compound oxide or compound hydroxide which comprisesat least one rare earth element selected from the group consisting ofLa, Ce, Pr, Nd, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
 3. Ahydrogen absorbing alloy composition according to claim 1, wherein therare-earth compound as the component (2) is at least one oxiderepresented by R¹ ₂ O₃ or at least one hydroxide represented by R¹ (OH)₃wherein R¹ is a rare earth element selected from the group consisting ofEu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
 4. A hydrogen absorbing alloycomposition according to claim 2, wherein the compound oxide is acompound represented by formula,

    (R.sup.2.sub.2 O.sub.3).sub.a.(R.sup.3.sbsp.2 O.sub.3).sub.b or (R.sup.2.sbsp.2 O.sub.3).sub.c.(R.sup.3.sbsp.2 O.sub.3).sub.d.(R.sup.4.sbsp.2 O.sub.3).sub.e,

and the compound hydroxide is a compound represented by formula,

    (R.sup.2 (OH).sub.3).sub.a.(R.sup.3 (OH).sub.3).sub.b or (R.sup.2 (OH).sub.3).sub.c.(R.sup.3 (OH).sub.3).sub.d.(R.sup.4 (OH).sub.3).sub.e,

wherein R², R³ and R⁴ are different from one another and each representsa rare earth element selected from the group consisting of La, Ce, Pr,Nd, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, a and b are each from0.1 to 0.9 by mole, provided that a+b is 1 by mole, and c, d and e areeach from 0.1 to 0.8 by mole, provided that c+d+e is 1 by mole.
 5. Ahydrogen absorbing alloy composition according to claim 3, wherein R¹ isa rare earth element selected from the group consisting of Yb, Er, Dyand Gd.
 6. A hydrogen absorbing alloy composition according to claim 4,wherein the compound oxide is a compound selected from the groupconsisting of (Yb₂ O₃)_(a).(Lu₂ O₃)_(b), (Yb₂ O₃)_(a).(Er₂ O₃)_(b), (Er₂O₃)_(a).(Dy₂ O₃)_(b), (Yb₂ O₃)_(c).(Sm₂ O₃)_(d).(Gd₂ O₃)_(e) and (Y₂O₃)_(c).(Er₂ O₃)_(d).(Yb₂ O₃)_(e), and the compound hydroxide is acompound selected from the group consisting of(Yb(OH)₃)_(a).(Er(OH)₃)_(b) and (Er (OH)₃)_(a).(Dy(OH)₃)_(b) wherein aand b are each from 0.1 to 0.9 by mole, provided that a+b is 1 by mole,and c, d and e are each from 0.1 to 0.8 by mole, provided that c+d+e is1 by mole.
 7. A hydrogen absorbing alloy composition according to claim2, wherein the compound oxide is a compound represented by formula R⁵MO_(m) wherein R⁵ is a rare earth element selected from the groupconsisting of Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, M is a metalelement except rare earth elements and m is a number of 2 to
 6. 8. Ahydrogen absorbing alloy composition according to claim 2, wherein thecompound oxide is a compound selected from the group consisting of Dy₂Hf₂ O₇, Eu₂ Hf₂ O₇, Yb₂ Zr₂ O₇, Er₂ Zr₂ O₇, YAlO₄, YNbO₄, YVO₄, ErAlO₃and Er₂ Hf₂ O₇.
 9. A hydrogen absorbing alloy composition according toclaim 1, wherein the oxides are oxides having a specific surface area of0.1 to 10 m² /g.
 10. A hydrogen absorbing alloy composition according toclaim 1, wherein the hydroxides are hydroxides whose rare-earth metalshave a purity of at least 30 weight %.
 11. A hydrogen absorbing alloycomposition according to claim 1, wherein the LnNi₅ hydrogen absorbingalloy is a LaNi₅ type hydrogen absorbing alloy of formula,

    Ln(Ni.sub.(w-x-y-z) Mn.sub.x Al.sub.y Co.sub.z),

wherein Ln is La alone or a mixture of La and another rare earthelement, 4.8≦w<5.3, 0<x≦0.6, 0<y≦0.5, and 0<z≦1.0.
 12. A hydrogenabsorbing alloy electrode for a nickel-hydrogen secondary battery, whichcomprises a hydrogen absorbing alloy composition according to claim 1and a conductive support.
 13. A hydrogen absorbing alloy electrode for anickel-hydrogen secondary battery, which comprises a hydrogen absorbingalloy composition according to claim 2 and a conductive support.
 14. Ahydrogen absorbing alloy electrode for a nickel-hydrogen secondarybattery, which comprises a hydrogen absorbing alloy compositionaccording to claim 3 and a conductive support.
 15. A hydrogen absorbingalloy electrode for a nickel-hydrogen secondary battery, which comprisesa hydrogen absorbing alloy composition according to claim 4 and aconductive support.
 16. A hydrogen absorbing alloy electrode for anickel-hydrogen secondary battery, which comprises a hydrogen absorbingalloy composition according to claim 7 and a conductive support.
 17. Acomposition comprising:a LnNi₅ type hydrogen absorbing alloy, wherein Lnrepresents at least one rare-earth element, and at least one rare-earthcontaining component selected from the group consisting of:oxides ofheavy rare-earth elements Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;hydroxides of heavy rare-earth elements Eu, Gd, Tb, Dy, Ho, Er, Tm, Yband Lu; compound oxides comprising at least two oxides of differentmetals including at least one rare-earth element; and compoundhydroxides comprising at least two hydroxides of different metalsincluding at least one rare-earth element.
 18. A secondary batteryhaving a hydrogen absorbing alloy composition comprising:a LnNi₅ typehydrogen absorbing alloy, wherein Ln represents at least one rare-earthelement, and at least one rare-earth containing component selected fromthe group consisting of:oxides of heavy rare-earth elements Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb and Lu; hydroxides of heavy rare-earth elements Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; compound oxides comprising at leasttwo oxides of different metals including at least one rare-earthelement; and compound hydroxides comprising at least two hydroxides ofdifferent metals including at least one rare-earth element.
 19. Ahydrogen absorbing alloy composition according to claim 17, wherein thecompound oxide is a compound represented by the formula

    (R.sup.2.sub.2 O.sub.3).sub.a (R.sup.3.sub.2 O.sub.3).sub.b or (R.sup.2.sub.2 O.sub.3).sub.c (R.sup.3.sub.2 O.sub.3).sub.d (R.sup.4.sub.2 O.sub.3).sub.e,

and the compound hydroxide is a compound represented by the formula

    (R.sup.2 (OH).sub.3).sub.a (R.sup.3 (OH).sub.3).sub.b or (R.sup.2 (OH).sub.3).sub.c (R.sup.3 (OH).sub.3).sub.d (R.sup.4 (OH).sub.3).sub.e,

wherein R², R³ and R⁴ are different from one another and each representsa rare-earth element selected from the group consisting of La, Ce, Pr,Nd, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, a and b are each from0.1 to 0.9 by mole, provided that a+b is 1 by mole, and c, d and e areeach from 0.1 to 0.8 by mole, provided that c+d+e is 1 by mole.
 20. Thesecondary battery having a hydrogen absorbing alloy compositionaccording to claim 18, wherein the compound oxide is a compoundrepresented by the formula

    (R.sup.2.sub.2 O.sub.3).sub.a (R.sup.3.sub.2 O.sub.3).sub.b or (R.sup.2.sub.2 O.sub.3).sub.c (R.sup.3.sub.2 O.sub.3).sub.d (R.sup.4.sub.2 O.sub.3).sub.e,

and the compound hydroxide is a compound represented by the formula

    (R.sup.2 (OH).sub.3).sub.a (R.sup.3 (OH).sub.3).sub.b or (R.sup.2 (OH).sub.3).sub.c (R.sup.3 (OH).sub.3).sub.d (R.sup.4 (OH).sub.3).sub.e,

wherein R², R³ and R⁴ are different from one another and each representsa rare-earth element selected from the group consisting of La, Ce, Pr,Nd, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, a and b are each from0.1 to 0.9 by mole, provided that a+b is 1 by mole, and c, d and e areeach from 0.1 to 0.8 by mole, provided that c+d+e is 1 by mole.